Solid and/or liquid particles suspended in a gas are referred to as aerosols, which are present naturally in the ambient atmosphere or as a result of man-made activities. Method and apparatus for measuring particles suspended in a gas are important for atmospheric aerosol research and in other scientific and technical disciplines where small particles suspended in a gas play a significant role.
An important method to measure the concentration and size distribution of aerosol particles is differential mobility spectrometry. For such a measurement the aerosol particles, i.e. particles suspended in a gas, must be conditioned in order to create a specific charge distribution on the particles. Particles not properly charge conditioned will give rise to erroneous results. The Wide-Range Particle Spectrometer (WPS™) manufactured by MSP Corporation (Product Information Bulletin, Model 1000XP, Wide Range Particle Spectrometer, MSP Corporation (2008)) is one such instrument capable of measuring aerosol size distributions from 0.01 μm to 10 μm in diameter. In this instrument, a sub-range of aerosol particle size from 0.01 to 0.5 μm is measured by differential mobility or scanning mobility spectrometry. For the purpose of this disclosure we will refer to both measurement approaches as differential mobility spectrometry, or DMS, since both are based on the differential mobility measuring principle. Instruments based on differential mobility spectrometry are available from several manufacturers. The electrical ionizer described in this disclosure can in principle be used with any one of these aerosol measuring instruments.
Traditionally, aerosol charge conditioning for DMS is accomplished by means of a radioactive ionizer. Two commonly used radioactive ionizers are Krypton 85 and Polonium 210 (B. Y. H. Liu and D. Y. H. Pui, “Electrical Neutralization of Aerosols, ” J. Aerosol Sci. 5:465-472 (1974) and B. Y. H. Liu, D. Y. H. Pui and B. Y. Lin, “Aerosol Charge Neutralization by Radioactive Alpha Source,” Particle Characterization, 3:111-116 (1986)). These radioactive ionizers make use of high-energy subatomic particles produced by radioactive decay to ionize the gas to form positive and negative ions needed for charge conditioning. Krypton 85 is a beta emitter, producing high-energy beta particles, i.e. electrons, by radioactive decay. Polonium 210 is an alpha emitter producing energetic subatomic alpha particles, which are the nuclei of helium atoms. These energetic sub-atomic particles then collide with gas molecules to form positive and negative ions for charge conditioning. These sub-atomic particles are much smaller than the size of a single atom, which is approximately 1.0 Å in the case of hydrogen. Alpha and beta particles are considerably smaller than 1.0 Å in size.
In comparison, aerosol particles are considerably larger. An aerosol particle with a diameter of 1.0 nm, which is 10 Å, is considered very small in aerosol studies and is near the lower size limit of particle measurement by DMS. Aerosol of such a small size is therefore much larger than particles of nuclear physics. Particles of nuclear physics are very different from the particles of interest in aerosol studies. These two types of particles are not the same and should be clearly distinguished. For the purpose of this disclosure, unless otherwise noted, the particles of interest are aerosol particles rather than sub-atomic particles of nuclear physics.
When gas containing suspended aerosol particles is exposed to energetic sub-atomic particles produced by radioactive decay, the gas becomes ionized to form positive and negative ions. The gaseous ions then collide with the suspended aerosol particles to produce a characteristic charge distribution referred to as a Boltzmann distribution (W. C. Hinds, Aerosol Technology, p. 303, Wiley (1982)),
  fn  =            exp      ⁡              (                              -                          n              2                                ⁢                                    e              2                        /                          ⅆ              kT                                      )                            ∑                  n          =                      -            ∞                          ∞            ⁢              exp        ⁡                  (                                    -                              n                2                                      ⁢                                          e                2                            /                              ⅆ                kT                                              )                    
where e is the elementary unit of charge, d is the particle diameter, k is Boltzamann's constant, T is the absolute temperature, n is the number of elementary units of charge on the particles and fn is the fraction of particles in the aerosol carrying n elementary units of charge. Table 1 shows the particle charge distribution according to the Boltzmann's law.
An aerosol in Boltzmann charge equilibrium will develop a charge distribution with substantially equal concentration of positively and negatively charged particles. The total charge on the particles, i.e. the sum of all positive and negative charges carried by the particles, is equal to zero. As a result, an aerosol in Boltzmann charge equilibrium has no overall net charge. Overall the aerosol is electrically neutral while the individual particles in the aerosol may carry a charge, although not all particles are charged. The conditions needed to produce Boltzmann charge distribution are discussed in B. Y. H. Liu and D. Y. H. Pui, “Electrical Neutralization of Aerosols,” 5. Aerosol Sei. 5:465-472 (1974) and B. Y. H. Liu, D. Y. H. Pui and B. Y. Lin, “Aerosol Charge Neutralization by Radioactive Alpha Source,” Particle Characterization, 3:111-116 (1986).
TABLE 1(from Hinds, 1982; page 302)Distribution of Charge on Aerosol Particles at Boltzmann EquilibriumParticle DiameterAveragePercentage of Particles Carrying the Indicated Number of ChargesμmCharge<−3−3−2−10+1+2+3>+30.010.0070.399.30.30.020.1045.289.6520.050.4110.619.360.219.30.60.100.6720.34.424.142.624.14.40.30.201.0000.32.39.622.630.122.69.62.30.30.501.644.66.812.117.019.017.012.16.84.61.002.3411.88.110.712.713.512.710.78.111.82.003.3320.17.48.59.39.59.38.57.420.15.005.2829.85.45.86.06.06.05.85.429.810.007.4735.44.04.24.24.34.24.24.035.4
When an aerosol carrying suspended particles are charge-conditioned by flowing the aerosol through a radioactive ionizer under suitable operating conditions, it will emerge from the ionizer carrying the charge distribution shown in Table 1. This specific charge distribution is then used for size distribution analysis by DMS.
An electrical ionizer for aerosol charge conditioning and measurement by DMS, therefore, must generate a charge distribution similar to the Boltzmann charge distribution generated by a radioactive ionizer in order to achieve accurate measurement results. One difference between radioactive ionizer and electric ionizer is that ionization by sub-atomic particles produced by radioactive decay occurs in the absence of an external electric field, whereas charge conditioning by ions generated by corona discharge frequently occurs when there is a significant electric field present. Not all electrical ionizers are thus capable of charge conditioning an aerosol to a sufficient degree to produce a Boltzmann distribution. As a result, an electrical ionizer capable of generating a charge distribution similar to the Boltzmann distribution is needed for high accuracy aerosol measurement by DMS. Such an ionizer is now needed because of the increased regulation on the use of radioactive material, which makes the use of radioactive ionizers less attractive or convenient for scientific research and technical applications.
Other developments in electrical ionizers include those described in F. J. Romay, B Y. H. Liu and D. Y. H. Pui, “A Sonic Jet Corona Ionizer for Electrostatic Discharge and Aerosol Neutralization” Aerosol Sci. Technol, 20: 31-41 (1994) and in U.S. Pat. No. 6,544,484. Both use a DC corona discharge to generate separate streams of positive and negative ions in clean air, which are then mixed with an aerosol to provide positive and negative ions for charge conditioning. The aerosol is thus diluted, which is a disadvantage in some applications. Both devices have failed to achieve wide spread acceptance, perhaps as a result of complexity, reliability, and/or cost. Another approach to aerosol charge neutralization is by means of an AC corona discharge as described by Riebel et al in U.S. Pat. No. 7,031,133.